1. Field of the Invention
This invention relates to a system for automatically monitoring gas sensors from a central location.
2. Description of the Prior Art
Industry has relied on gas detectors for the protection of life and property. Gas detectors typically use remote sensors operating on the principal of diffusion so that the presence of any hazardous gas, flammable or toxic, may quickly be detected at a remote location of a facility or process. The presence of such gas is reported to a control room. The concentration of the gas is reported and an alarm is activated when the concentration exceeds certain preset values.
Conventional prior art sensors are individually wired to the field location from the central control panel with whatever number of wires are required to deliver the power and return the information. This is usually a minimum of three to five wires for each sensor.
The major problem in any detection and alarm system is reliability. To be safe the operator must frequently check and calibrate each sensor. Calibration must be done quite frequently in order to assure reliability. For hazardous vapors the standard in the industry is catalytic combustion. Such sensors are known to loose their ability to detect flammable vapors over time and therefore become insensitive to the presence of hazardous gas and fail to produce an adequate alarm at the central monitoring panel.
A number of attempts have been made to reduce the number of wires connecting the sensors in the field and to control the cost of installation.
It is possible to employ multiple wires to each remote sensor to deliver the various electrical instructions that are required to calibrate a sensor from a central monitor panel. Such multiplex wire systems control all the various functions of zero, span, power and signal return. An operator manipulates all the functions from a central panel and knows if a particular sensor is accurate and reliable. Usually the system is calibrated by two people. One operator provides the sensor with a test gas and another operator at the central panel notes the reading and makes the appropriate adjustment. This arrangement produces acceptable reliability if it is done frequently enough. Many users fail to make calibrations frequent enough to detect sensor calibration problems. In order to reduce the cost of manpower for calibration purposes, various instruments have been developed so that equipment may be calibrated by one man.
There are several different types of sensing systems disclosed in the prior art literature. Such systems are found in the patent literature as well as in general commercial literature. The following prior art devices and systems appear to be relevant and typical of that prior art.
U.S. Pat. No. 4,388,822 discloses a microprocessor controlled gas detector system which requires three wires to function. A probe converts the signals from analog to digital ratiometric and is responsive to a reset pulse generated by the microprocessor. The reference discloses that it is known to use a microprocessor to control a probe which produces a signal in response to the detection of a gas.
U.S. Pat. No. 4,390,869 discloses a gas detection system in which a plurality of sensing units are connected together in parallel. This appears to be a four wire type system. The sensing units can be grouped in various different ways to obtain various different gas detection alarm data. Moreover, the sensors are arranged so that a malfunction in the unit itself can be signalled to a central station.
U.S. Pat. No. 4,290,055 is of general interest in that it discloses a detector system, primarily for fire, connected by two pair of wires. The wire carrying the interrogatory pulse to each detector is connected serially to said detectors. U.S. Pat. No. 4,290,055 is not directed towards gas detection. However, it is of some relevance in that it teaches sequential sampling of serially connected detection stations by a scanning mechanism.
U.S. Pat. Nos. 3,710,372 and 4,359,721 disclose serially connected detection systems wired to allow the sensing of individual detector units. For example, U.S. Pat. No. 4,359,721 discloses a system in which a frequency divider provides for time sampling of a serially connected sensor. The output is demodulated in order to detect the values sensed by the sensors.
U.S. Pat. No. 4,384,925 is of interest in that it teaches a microprocessor controlled gas sensing system which automatically controls a solenoid valve to feed a standard calibration gas to a sensor at preset levels.
Similarly, U.S. Pat. No. 4,151,738 discloses another automatic gas calibration system. A zero gas is sampled continuously except during sampling or recalibration operation and a standard gas is sampled periodically and automatically during the recalibration mode.
U.S. Pat. No. 3,481,179 is of interest only in that it discloses a gas detection sensor head with an annular channel around the sensor for feeding and calibration gas. The device further includes a means for evenly distributing radial flow of calibration gas. This device comprises a sintered bronze ring which resists the gas flow.
U.S. Pat. No. 4,305,724 is assigned to Delphian Partners. Delphian Partners is known to use a computer type instrument. The sensors are typically analog in that they transmit analog signal to a computer using a four-20 milliamp proportional signal. Calibration is done in the field by signalling from the sensor to the CPU with a magnetic type switch. The sensor contains no intelligence, but the computer does take the information of "zero" and "span" generated by the man in the field so that the computer ends up with "zero" and a "span" number which it can use to compute a reading. The operator in the field has to manually connect or cover the sensor to introduce the test gases and he also has to manually operate a magnetic device referred to by Delphian as a "Sensical" in order to make the system work properly.
The Bendix Corporation employs a two wire sensing device including two wires to carry power. On the power wires it communicates the information about the sensors using a high frequency carrier. The carrier is modulated with coded sensor information. In order to calibrate the sensor the operator in the control room sets the module in the calibrate mode. At the sensor, zero and test gases must be added by the operator in the field. The microprocessor knows to hold the peak reading because the operator pushes a button on the sensor synchronizing the reading with the test gas. The value of the test gas is digitally entered into the module and the sensor peak signal is calibrated to the test value. Returning the module to normal establishes the new calibration. Accordingly, two men are required to operate and calibrate the system or one man traveling to and from the control room to the sensor. Insofar as understood the Bendix system does not provide for the transmission back and forth of intelligence between the control module and the sensor. Each sensor is wired individually to the control unit.
International Sensor Technology offers a system that requires the operator to initiate testing so that the computer will adjust the "zero" and the "span" settings. The sensor can be calibrated by a man in the field or remotely through a sample tube. The system only uses the sample tube for calibration gas. Each sensor is analog and therefore transmits an analog 4-20 milliamp signal to the central control unit. There appears to be no intelligent communication between the sensor and the control section. The computer is basically a data logger with the ability to digitize the information from the sensors.
Rexnard has a multichannel unit that sequences eight sensors. Each sensor is connected by a series of wire. The information is transmitted by a voltage-to-frequency method. All sensors contain a "zero" and "span" mechanism. They are field calibrated by opening the explosion proof housing, plugging a meter into it, introducing the "zero" and the "test" gases and making the appropriate adjustments at the sensor.
Honeywell, Inc. has a differential transmitter known as the ST3000 Smart Transmitter which may be used in the field to remotely configure, re-range, diagnose and display process values. The device is primarily intended to measure and control process flow, liquid level and pressures in a system. The device is unusual in that it can be inserted at any place within the circuit loop and interrogate remote sensors at any location within that loop. However, the device appears to be otherwise unrelated to the hazardous gas sensor of the present invention.
MSA Instruments has an instrument known as the DAN.TM. 6000 Data Acquisition Network for sensing combustible gas at remote locations. The device is capable of turning on a zero and span gas in the field. Calibration is done in the field by an operator who uses a magnet to trigger the calibration procedure. The procedure is a timed sequence-zero and then span gas. If the results of the calibrations are within the time window and if the deviation is .+-.10% then the device automatically corrects itself and prints out the values. If the results are outside of the .+-.10% value then the device must be corrected by the man in the field. The device can accomodate up to 14 remote station monitors with up to 16 sensors each. MSA also provides a calibration check kit including two three liter cans of gas, a balloon designed to hold one-half liter of gas, and the appropriate connections and clamps for introducing the gas to the detectors. The manual procedure necessary for testing the sensor appears to be relatively complicated and requires the presence of an individual in the field under possibly hazardous conditions.
Insofar as understood, none of the prior art systems teach a simple digital two wire mechanism for reliably connecting sensors to a control unit. The system disclosed herein is a true digital system which has extreme reliability because of its unique method of operation.